Detection of a Type IIn Supernova in Optical Follow-up Observations of IceCube Neutrino Events

TL;DR

This paper reports the detection of a Type IIn supernova in optical follow-up observations of IceCube neutrino events, analyzing its properties and assessing the likelihood of a causal connection with neutrino signals.

Contribution

It presents the discovery and detailed analysis of a supernova coincidentally near a neutrino alert, and evaluates the potential link between neutrino emissions and the supernova.

Findings

The supernova PTF12csy is unlikely to be causally connected to the neutrino alert.

No significant long-term neutrino signal was detected over a year.

The supernova shows signs of ejecta interaction with dense circumstellar medium.

Abstract

The IceCube neutrino observatory pursues a follow-up program selecting interesting neutrino events in real-time and issuing alerts for electromagnetic follow-up observations. In March 2012, the most significant neutrino alert during the first three years of operation was issued by IceCube. In the follow-up observations performed by the Palomar Transient Factory (PTF), a Type IIn supernova (SN) PTF12csy was found $0.2^\circ$ away from the neutrino alert direction, with an error radius of $0.54^\circ$. It has a redshift of $z=0.0684$, corresponding to a luminosity distance of about $300 \, \mathrm{Mpc}$ and the Pan-STARRS1 survey shows that its explosion time was at least 158 days (in host galaxy rest frame) before the neutrino alert, so that a causal connection is unlikely. The a posteriori significance of the chance detection of both the neutrinos and the SN at any epoch is $2.2 \, \sigma$ within IceCube's 2011/12 data acquisition season. Also, a complementary neutrino analysis reveals no long-term signal over the course of one year. Therefore, we consider the SN detection coincidental and the neutrinos uncorrelated to the SN. However, the SN is unusual and interesting by itself: It is luminous and energetic, bearing strong resemblance to the SN IIn 2010jl, and shows signs of interaction of the SN ejecta with a dense circumstellar medium. High-energy neutrino emission is expected in models of diffusive shock acceleration, but at a low, non-detectable level for this specific SN. In this paper, we describe the SN PTF12csy and present both the neutrino and electromagnetic data, as well as their analysis.